This invention relates to the pasteurization of dairy products such as milk, cream, and ice cream mix and other liquid food products, e.g., fruit juices and soups. The invention is more particularly concerned with a continuous-process pasteurization technique for heating milk (or other food product) to an ultra high temperature suitable for long shelf life, and which recovers most of the heat employed by regeneration, and where the pasteurized product is cooled by incoming raw product, which in turn is heated to a temperature near the pasteurization temperature. The invention is also concerned with a technique which avoids a cooked or scalded taste in the food product.
Unprocessed milk and other raw food commodities are highly perishable, and also may harbor pathogenic microorganisms. Milk is a highly nutritious food, and thus also serves as an excellent growth medium for microorganisms, most of which are capable of deteriorating or spoiling the milk or milk products. This is the case also for fruit juices, broths, and many other liquid food products. Some microorganisms are pathogenic, and can present a threat to public health, and so it is important that the product is rendered completely free of pathogenic microorganisms. Therefore, milk and other liquid food products are processed to make them safe for human consumption and to reduce spoilage to the lowest level possible.
Pasteurization is a process for heat treating the milk or other food process to kill these microorganisms. For milk, pasteurization requires that every particle of milk or milk product be heated to a temperature, for an adequate length of time sufficient to render it free of pathogens, most notably Rickettsia and Mycobacterium tuberculosis. The U.S. Public Health Service has published standards for time-temperature relations for pasteurization of milk and other milk products, and these can be at higher temperatures or longer times for products that may contain added sweeteners or additional milk fat. Most modern dairies employ a continuous process pasteurization technique, rather than a batch process, even for highly viscous products such as ice cream mix. Typically, for most pasteurized milk, other than ultra-pasteurized or UHT pasteurized, a continuous process high-temperature, short time (HTST) technique is used, in which the milk is heated to about 165 degrees F. and then passes through a holding tube where the milk is held at that temperature for a short time, e.g., sixteen seconds. These HTST pasteurizers can employ either a plate heat exchanger, or “press,” in which parallel plates define flow channels for the milk and for heating and cooling media, or can employ a tubular heat exchanger in which two or more tubes of different diameter are arranged coaxially to define flow paths for the milk and other heat transfer media.
In a HTST set up, cold raw milk (about 40° F.) is supplied from a balance tank into a regenerator section of the pasteurizer. Here the raw milk is heated up by heat given up by pasteurized milk flowing in counter current direction through an opposite side of the regenerator. The raw milk leaves the regenerator and passes through a positive displacement timing pump that delivers the milk under pressure to further stages. The pre-heated raw milk is then forced through a heater section where a heating medium, e.g., water or a food grade synthetic product, heats the milk up to a temperature of at least about 162° F. The milk, having reached this pasteurization temperature, flows through a holding tube or timing tube, where the milk is held at this temperature for a predetermined time, e.g., at least 16 seconds. The velocity of the milk product is determined by the speed of the timing pump, the diameter and length of the holding tube and other sources of surface friction. After passing temperature sensors at the end of the holding tube, the milk flows past a flow diversion device, which is intended to return the milk product through a divert line to the balance tank if the temperature of the product is below the preset pasteurization temperature. Properly heated milk will continue forward, and passes through the pasteurized side of the regenerator where it is cooled by the incoming raw milk. The milk can be cooled further to about 40° F., and processed for bottling, packaging, cheesemaking, or other use. A homogenizer may be used as the timing pump just described, or may constitute additional auxiliary equipment, typically at the stage where the milk or milk product has been heated. Booster pumps are also present in the flow path to ensure correct pressure and flow relationships. For any continuous pasteurization technique, it is important to maintain a higher pressure on the pasteurized side of any product-to-product heat exchanger, such as the regenerator, so that in the event there is any pinhole leak the flow of milk is away from the pasteurized side. This prevents contamination of the pasteurized milk with raw milk. The pressure differential is maintained using the timing pump, and other pumps and pressure controllers, and by ensuring that there is any vertical rise in the product flow path is kept within limits.
The purpose of the regenerator is to save energy used in heating and cooling the food product during pasteurization, by using the heat content of the pasteurized product to pre-heat the incoming cold raw milk. The efficiency, i.e., percent regeneration, is equal to the ratio of the temperature increase in the raw milk due to regeneration to the total temperature increase. For example, for cold milk drawn from the balance tank at 40° F., heated through the regenerator to 145° F., and then heated to a final temperature of 165° F., the efficiency would be 84%, i.e., an 84% regeneration:
            (              145        -        40            )              (              165        -        40            )        =            105      125        =          84      ⁢      %      
As the cost of energy is a significant consideration in the overall cost of processing the milk product, it is desirable to keep the amount of-regeneration as high as possible, and thus to reduce the cost of adding heat at the heater stage.
Ultra high temperature treatment of the food product, i.e., UHT pasteurization, involves heating the liquid food product continuously, and ensuring that every particle of the milk or other food product has been held at the predetermined ultrahigh temperature for a minimum length of time. The UHT technique can be incorporated into a sterilization technique, in which the product is heated to a temperature of 240° F. or above, and is held for a corresponding holding time to ensure that the microorganisms and their spores in the product are destroyed. Then the sterilized product is packaged aseptically, and aseptically sealed. The intention here is to permit the liquid food product to be stored at room temperature indefinitely without spoilage due to action of microorganisms. However, the process of ultra high pasteurization processing may alter the flavor or desirable color or texture of the product, and may result in a “cooked” or scalded flavor in the product.
A vacuum treatment is sometimes employed to remove as much of the undesirable flavor components as possible from the product. In the vacuum process, milk is first heated to the desired temperature, and then is passed into a chamber in which the pressure has been reduced by a partial vacuum. The pressure in the chamber is low enough to cause the volatile flavor components to vaporize, and these are then evacuated from the chamber. Some of the water in the product may be evaporated as well. The vacuum treatment will also reduce any flavor components that result from the cows' ingestion of weeds or flavor-producing feed components.
In addition to the public health and spoilage issues addressed by pasteurization, it has been discovered that some proteins and enzymes in milk that will bring about chemical changes in the stored product can be altered or removed, i.e., denaturized, by heat treatment.
A continuous flow milk sterilization process is described in U.S. Pat. No. 3,567,470, in which raw milk is passed though counterflow regenerators to a pasteurizer and then is passed though counterflow heat exchangers where its temperature is raised to a sterilization temperature. However, in this process, there is a rather high temperature differential between the milk and the heating medium in the ultra high temperature heat exchangers used for sterilization, so that the temperature differential between the milk and the heating medium at any given point of reference is always above 20 degrees and may reach 40 degrees F. This means that the heating medium (steam) entering the heat exchanger has to be 300° F. to raise the milk to a sterilization temperature of 270 to 280° F., and the medium leaving is at 280° F. where it meets the product entering at 240 to 260° F. The processed milk in this system has to pass through a pair of vacuum tanks to remove air and entrapped volatile gases in an attempt to remove the objectionable burnt flavor that is associated with the sterilized milk.
Homogenization is typically employed to break up the butterfat globules so that they will remain in suspension in the aqueous portion of the milk or other dairy product. A homogenizer is employed for this purpose, which is universally placed at the HT phase of the pasteurizer, i.e., where the milk or other dairy product has been heated to the temperature of 175° F. The homogenizer consists of a pump where pistons move the milk at a prescribed flow rate and raise the pressure to several thousand PSI, and a screen, orifice, or equivalent means which the milk product is forced through to break up the fat globules. The homogenizer is placed at the 175° F. stage, even when the product is being heated again up to 280° F. in an ultrapasteurization or UHT pasteurization arrangement. This raises the pressure to a high level in the stage immediately following the homogenizer. This can have an adverse affect on the operation of the diversion valves which are designed for pressures of only about 150 psi. Also, in a UHT arrangement, this produces a high pressure on the raw side of the regenerator stage that raises the milk product temperature from 175° F. to 260° F. In order to ensure that the pasteurized product on the return side of the regenerative heat exchanger is at a higher pressure than the product on the raw or inflow side, a booster pump and flow meter have to be installed at the 280° F. stage. This additional equipment adds an increased capital cost to the system, and also requires additional energy to operate. Moreover, placing the homogenizer at the midway point of the regenerator limits the amount of regeneration that is available.
Moreover, the homogenizer is typically placed after the heater but before the legal hold tube and before the flow diversion valve. In a UHT arrangement, the milk temperature has to be elevated from the HT temperature (162° to 175° F.) up to 280° F., and there has to be substantial pressure to pump it through the second regenerator, heater, and the sterile side of the second regenerator and the pasteurized side of the first regenerator and final cooler. The pressure on the return side has to be significantly higher than on the inflow side, so that if there is any leak or pinhole in the product-to-product regenerator, the leakage will be into the raw side and not into the pasteurized or sterile side. The homogenizer employs a flow pump that creates great enough pressures to pass the product through an orifice to reduce the size of the fat globules in the product. The legal flow diversion valve is spring actuated, and if the pressure exceeds about 150 psi, the valve will not function reliably.
By taking the milk from the flow diversion valve directly to the homogenizer, this problem can be alleviated and higher pressures, higher velocities, and thus higher head transfer rates can be attained. A vacuum breaker can be installed, if necessary, ahead of the homogenizer so that in the event that the homogenizer is unable to induce flow, that would not impact the sixteen-second legal hold time in the hold tube.
For a reasonable rate of heat exchange efficiency, which means a reasonable requirement for heat exchange surfaces, velocities of about six feet per second or higher are desirable. With average velocities of six feet per second, it is possible to attain regeneration efficiencies of 85% and stay below the 150 psi limit of the flow diversion valves. By placing the homogenizer after the diversion valve, the average velocities of well over six feet per second can be achieved by proper selection of tube sizes, and a regeneration efficiency of 90% can be attained. This represents a significant energy savings over the maximum of 85% mentioned above. The surface area requirement for a regenerator of 90% efficiency is about 1.6 times that of a regenerator of 85% efficiency, with the pressure heads being related to the square of the velocities. However, with the homogenizer positioned after the flow diversion valve, these higher pressures, which can be 400 to 500 psi, can be achieved.
In a UHT pasteurizer system, homogenization is carried out at. approximately 175° F. Some systems homogenize the milk at a midpoint stage of the regenerator, and then continue heating the milk to the UHT temperature. Other systems homogenize the sterile or ultra-pasteurized milk (which has reached the UHT temperature 280° and has been held for two seconds), and which has been cooled back down to 175 to 180° F. Because homogenization is done as part of the pasteurization process in a continuous pasteurization system, and because a disagreeable off flavor is sometimes present where homogenization takes place under 135° F., homogenization has come to be universally carried out either after the regenerator at 145° or after the HT heater at 163° to 175°. Homogenization systems employ a piston-driven pump, which has to be able to generate sufficient pressures to drive the milk or other product though the orifice or screen to break up the milk fat globules. The term orifice is used to mean any arrangement of fine openings or the like that results in diminution of the droplet size or globule size of the butterfat portion so that it remains in suspension in the aqueous portion. The pressure from this pump can be up to a few thousand psi. The homogenizer pump acts to pump the dairy product at a steady volume rate. The action of the homogenization system can increase the temperature of the product by 4° to 5°. In a UHT pasteurization system, the presence of the homogenizer at the intermediate temperature stage of the regenerator on the raw side means that an additional pump is needed at the high temperature stage to ensure that the pressure on the return or pasteurized side of the regenerator is higher than on the raw products side.